lecture 6

Question 1

how plastic defornation occurs for metals

Answer 1

by the movement of many dislocations - process is called slip

Question 2

dislocation motion

Answer 2

Under applied shear stress, dislocations can move by breaking bonds
consecutively (rather than simultaneously) è Requires less energy

so
depends on incrementally breaking bonds

Question 3

fot what are fdislocations imp

Answer 3

for mech properties of metals but not ceramics

Question 4

slip

Answer 4

• Movement of dislocations under applied stress is called

Question 5

slip direction

Answer 5

Direction of dislocation movement i

Question 6

slip plane

Answer 6

• Plane swept out by the dislocation is called the

Question 7

slip system

Answer 7

Combination of direction and plane

Question 8

The number of dislocations is expressed as

Answer 8

The number of dislocations is expressed as Total dislocation length per
unit volume (mm/mm3 which becomes mm-2)

Question 9

density of disln

Answer 9

increases with deformation since . more dislocations are created during plastic
deformation

Question 10

Dislocations & Materials Classes
* Metals:

Answer 10

• Disl. motion easier.
• non-directional bonding
• close-packed directions for slip

Question 11

*Dislocations & Materials Classes
Covalent Ceramics (Si, diamond):

Answer 11

Motion hard
* directional (angular) bonding

Question 12

Dislocations & Materials Classes
Ionic Ceramics (NaCl):

Answer 12

• Motion hard.
• need to avoid ++ and – neighbors

Question 13

When metal is plastically deformed, most of energy

Answer 13

is dissipated as heat

Question 14

Strain fields of dislocations

Answer 14

Strain fields of dislocations can
affect other dislocations.
Same sign dislocations on same
plane repulse each other:
whereas opposite signs will attract
and annihilate on meeting.

Question 15

fcc bcc slip sstms

Answer 15

FCC & BCC metals tend to have high numbers of slip systems (³ 12) so
these metals tend to be ductile - deformable

Question 16

HCP slip sstms

Answer 16

HCP metals have few active systems so are normally brittle

Question 17

Slip occurs by shear on preferred planes in preferred
directions (closest packed).
SUMMARY
* Dislocation movement depends on the type of crystal structure:
Dislocation move easier in FCC than BCC. Motion is difficult in
most HCP metals.